The present invention relates generally to the field of mineral separation and in particular to a flotation process for depressing arsenic minerals using the synergistic combination of a polyamine, a sulfur containing species, and oxidation.
The production of most metals proceeds in two steps. First, a metal compound is concentrated from an ore, which is mostly an oxide or a sulfide. Second, the metal concentrate is smelted and refined.
The first step in producing metals is breaking apart the ore by crushing and grinding, and separating particles of metal minerals from the gangue. Gangue is a general term for valueless minerals which are mined together with the valuable minerals.
The separation of a metal mineral from the gangue is most commonly achieved by a process called flotation. The mineral particles are suspended in a fluid in a tank under agitation. Air is forced or sucked into the suspension and broken into air bubbles. The valuable metal mineral particles become attached to the air bubbles and float (hence the name “flotation”) to the surface, forming a froth, which can be skimmed off. The gangue particles are not attached to the air bubbles and are discharged at the bottom of the tank.
Complete selectivity with respect to the separation of the individual minerals is impossible to achieve and often impure concentrates are produced.
It is known to add other chemical reagents to improve the selectivity of the separation process. One class of such reagents are the so-called depressing agents known to reduce the flotation rate of gangue minerals. Depressants affect the flotation process by rendering the unwanted mineral hydrophilic (i.e. water wettable), thus reducing the possibility of the unwanted mineral being floated simultaneously with those substances which are to be concentrated in the froth.
The concentrates need further processing or refinement in subsequent treatment steps to extract metals by high temperatures or chemical processes. Roasting, converting and smelting remove iron, sulfur and other impurities. The ore is heated in oxygen or air. The sulfur combines with oxygen and is blown off as gas. The remaining metal oxide must be further refined and purified.
Arsenic containing minerals are sometimes found in close association with base and precious metal minerals and, as a result, the co-mining of arsenic with metal minerals is inevitable. Mines may produce tailings with high residual arsenic concentrations due to the presence of arsenic in the ore. Mining of arsenic-bearing ores with the consequent oxidation of sulfides and release of metals and metalloids produces considerable contamination potential. Arsenic can be a by-product of smelters and coal or waste combustion.
If arsenic minerals are floated with metal minerals into the concentrates, they will be carried over to the subsequent pyrometallurgical processes. This creates two issues: smelters may constitute a major source of arsenic emissions from operations which pyrometallurgically treat sulfide concentrates containing arsenic. This is a major environmental concern. The other is the detrimental effect of arsenic on the metallurgical performance of the pyrometallurgical processes (Jackson, Nesbitt, Scaini, Dugal and Bancroft, Gersdorffite (NiAsS) chemical state properties and reactivity toward air and aerated, distilled water, American Mineralogist, vol. 88, pp. 890–900, 2003). It is often important that arsenic minerals are depressed during flotation of metal minerals so that the former are not carried over to the pyrometallurgical processes. This requires effective arsenic depressants to be added during flotation.
Nickel mining is particularly affected by high arsenic content. Nickel occurs in a number of minerals; the most economically important being pentlandite (nickel-iron sulfide) while violarite, millerite and garnierite (nickel-magnesium silicate) are also of importance. Pentlandite almost always occurs with much larger quantities of pyrrhotite (Fe7S8) which may contain a small fraction (up to 1%) of nickel but every effort is made to reject this mineral to tailings. Nickel is obtained commercially from pentlandite of the Sudbury region in Ontario, which produces about 30% of the world's supply of nickel. In Sudbury, nickel and copper sulfide minerals are concentrated by the flotation process into a Cu—Ni bulk concentrate, then smelted and converted to give sulfur dioxide, fayalite (iron silicate) slag and a Cu—Ni matte. The two metals are then separated from each other using the matte separation process. Mineral separation of Ni—Cu ores from the Sudbury region is discussed in greater detail in U.S. Pat. No. 5,411,148.
Arsenic occurs in various mineral forms, such as arsenides in sulfide minerals and as arsenate. One of the most common arsenic containing minerals is arsenopyrite (FeAsS). In the weathering of sulfides, arsenic can be oxidized to arsenite and arsenate. Arsenic oxide is also formed as a by-product of copper, lead and nickel smelting. The toxic nature of arsenic and its compounds presents a large concern for the environment. It has been found that certain ore bodies in the mines of the Sudbury region have arsenic content up to 200 times the normal content. Blending the ore into the feed to the mill has, at times, resulted in an increase in arsenic content of the Cu—Ni bulk concentrate to a level that significantly affects smelters and, more importantly, the efficiency of Cu—Ni separation in the matte separation plant. In the Sudbury region, the arsenic mainly occurs in a sulfide mineral with nickel called gersdorffite (NiAsS), with a small amount being in the form of cobaltite (CoAsS).
It is known in the prior art to use oxidation in combination with a magnesium chloride (MgCl2.6H2O), ammonium chloride (NH4Cl) and ammonium hydroxide (NH4OH) reagent (MM) as a depressant for arsenic-containing minerals during flotation of base and precious metal sulfides (Abeidu and Almahdy, Magnesia Mixture as a Regulator in the Separation of Pyrite from Chalcopyrite and Arsenpyrite, International Journal of Mineral Processing, vol. 6, pp. 285–302, 1980; Yen and Tajadod, Selective Flotation of Enargite and Chalcoyrite, Flotation Kinetics and Modelling, pp. B8a49–B8a55, 2000; Tapley and Yan, The Selective Flotation of Arsenopyrite from Pyrite, Minerals Engineering, vol. 16, pp. 1217–1220, 2003). However in the case of ores containing pyrrhotite as a gangue sulfide, the oxidation step results in the activation of pyrrhotite flotation and consequently a low grade concentrate of the valuable metal.
Depression of pyrrhotite during the flotation of Ni/Cu minerals has been achieved by using polyamines such as ethylene diamine (EDA), diethylenetetramine (DETA) and triethylenetetramine (TETA) as described in U.S. Pat. No. 5,074,993, or in combination with sodium sulfite or other sulfoxy species with sulfur valence less than 6 as described in U.S. Pat. No. 5,411,148.
WO 98/0858 teaches that TETA may be used against a large array of minerals including arsenides in a leaching process. A two-component, aqueous chemical leaching solution is taught, comprising any suitable oxidizing agent such as hydrogen peroxide, and any suitable chelating agent such as TETA. However, the use of TETA in a process of flotation and depression of NiAsS is not disclosed.
U.S. Pat. No. 4,681,675 discloses flotation utilizing 3-hydroxytrimethylene sulfides as depressants for iron, nickel, copper, lead, and/or zinc minerals, such as niccolite (NiAs) and tennantite ((Cu,Fe)12As4S13).
U.S. Pat. No. 2,805,936 teaches autoclave leaching of non-ferrous metals, particularly nickel and arsenic using nitric acid.
There is a general need in the field of metal recovery for depressing arsenic content. There is also a particular need in the fields of nickel and copper mining for a process of depressing pyrrhotite and arsenic while producing a high grade concentrate of the desired valuable nickel and copper metal such as pentlandite (FeNiS) and chalcopyrite (CuFeS2).